Unit test refactoring

[LinZhihao-723]

Request

The current unit tests have the following limitations:

• All unit tests are located in the same test file
• The base class is not designed to work with key-value pair log events, as introduced in PR feat: Add ClpKeyValuePairStreamHandler to support logging dictionary type log events into CLP key-value pair IR format. #46, causing code duplication to test key-value pair log event handlers
• The unit test uses readers from this repo to deserialize log events from IR stream. These methods are supposed to be deprecated: Deprecate reader implementation #45
• Variable/Method naming doesn't use _xxx to indicate private/protected members in test classes

Possible implementation

• Split files into their dedicated files
• Redesign the base class for testing both unstructured and structured log events
• Replace the reader implementation with the ones from clp-ffi-py
• Apply _ prefix to private/protect members in test classes